1. Field of the Invention
The present invention relates to a bonding method and more particularly to a wire bonding method which is suitable for devices in which the height difference between the first and second bonding points is small and the wiring distance is short.
2. Prior Art
FIGS. 4(a) and 4(b) show prior art semiconductor device assembly bonding processes.
In these processes, a pad 2a (which is a first bonding point) on a semiconductor chip 2 mounted on a lead frame 1 and a lead 1a (which is a second bonding point) on the lead frame 1 are connected by a wire 3; and FIG. 4(a) shows a triangular wire loop (of the bonded wire 3), and FIG. 4(b) shows a trapezoidal wire loop.
These bonding processes are disclosed in, for example, Japanese Patent Application Laid-Open (Kokai) No. H4-318943 and Japanese Patent Application Publication (Kokoku) Nos. H1-26531, H5-60657 and H6-101490.
In particular, Japanese Patent Application Laid-Open (Kokai) No. H4-318943 discloses both triangular and trapezoidal loops in FIG. 3 and in column 1, lines 31 through 41. Japanese Patent Application Publication (Kokoku) No. H1-26531 discloses a triangular wire loop in FIG. 3(c) and in column 3, line 3 through line 25. Japanese Patent Application Publication (Kokoku) No. H5-60657 discloses a triangular wire loop in FIG. 2(e) and in column 3, line 30 through column 4, line 40. Japanese Patent Application Publication Kokoku) No. H6-101490 discloses a trapezoidal wire loop in FIG. 1 and in column 4, lines 39 through column 5, line 22.
The triangular loop of FIG. 4(a) is formed by the process shown in FIG. 5(a) and FIG. 6. As seen from FIG. 5(a), a capillary is moved from point A to G through points B, C and F to form the triangular wire loop.
More specifically, as shown in FIG. 6, in step (a), the capillary 4 is lowered with a clamper (not shown) which holds the wire 3 maintained in an open state, and a ball formed on the tip end of the wire 3 is bonded to the first bonding point A, after which the capillary 4 is raised to point B, delivering the wire 3. Next, in step (b), the capillary 4 is moved horizontally to point C in the opposite direction from the second bonding point G. A loop forming operation in which a capillary is moved in the opposite direction from a second bonding point is generally called a "reverse operation". As a result of this reverse operation, the wire 3 assumes a shape that is inclined from point A to point C, and a kink 3a is formed in one portion of the wire 3. The wire 3 delivered in this process and extending from point A to point C forms a neck height part 31 shown in FIG. 4(a).
Next, in step (c), the capillary 4 is raised to point F, delivering the wire 3; and the clamper (not shown) is closed. When the clamper is closed, no wire 3 is delivered from the capillary 4 even if the capillary 4 is subsequently moved. Next, in step (d), the capillary 4 is positioned at the second bonding point G by being moved circularly or by being lowered after a circular-arc movement, thus bonding the wire 3 to the second bonding point G.
On the other hand, the trapezoidal loop shape shown in FIG. 4(b) is formed by the process shown in FIG. 5(b) and FIG. 7. As seen from FIG. 5(b), the capillary is moved from point A to G through points B, C, D, E and F.
More specifically, as shown in FIG. 7, in step (a), the capillary 4 is lowered with the clamper (not shown) which holds the wire 3 maintained in an open state, and the ball formed on the tip end of the wire is bonded to the first bonding point A, after which the capillary 4 is raised to point B, delivering the wire 3. Next, in step (b), the capillary 4 is moved horizontally to point C in the opposite direction from the second bonding point G. As a result, the wire 3 assumes a shape that is inclined from point A to point C, and a first kink 3a is formed in one portion of the wire 3. The wire 3 delivered in this process and extending from point A to point C forms the neck height part 31 shown in FIG. 4(b).
Next, in step (c), the capillary 4 is raised to point D, delivering the wire. Afterward, in step (d), the capillary 4 is again moved horizontally to point E in the opposite direction from the second bonding point G, i. e., a reverse operation is performed. As a result, the wire 3 assumes a shape that is inclined from point C to point E, and a second kink 3b is formed in one portion of the wire 3. The wire 3 delivered and extends from point C to point E forms the trapezoidal length part 32 shown in FIG. 4(b).
Next, in step (e), the capillary 4 is raised to point F, delivering an amount of wire 3 that forms the inclined part 33 shown in FIG. 4(b); and the clamper (not shown) is closed. When the clamper is closed, no further wire 3 is delivered even if the capillary 4 is subsequently moved. Next, in step (f), the capillary 4 is positioned at the second bonding point G by being moved circularly or by being lowered after a circular-arc movement, thus bonding the wire 3 to the second bonding point G.
The triangular loop formation process shown in FIG. 5(a) and FIG. 6 is advantageous in that the loop can be formed by a simpler process than the trapezoidal loop formation process shown in FIG. 5(b) and FIG. 7, and the loop formation is accomplished in a shorter time. However, in cases where the height difference between the first bonding point A and the second bonding point G is large, or in cases where the first bonding point A and the end portion of the semiconductor chip 2 are separated from each other by a considerable distance, the wire 3 contacts the semiconductor chip 2 if the wire is in the triangular loop shape as shown in FIG. 4(a). In such cases, contact between the wire 3 and the semiconductor chip 2 is prevented by using the trapezoidal loop shape as shown in FIG. 4(b).
As seen from the above, either a triangular loop or a trapezoidal loop is selected depending on the conditions involved. However, in the case in which the height difference between the first bonding point A and the second bonding point G is small (e.g., 100 .mu.m or less), and the wiring distance is short (e.g., 1 mm or less), problems arise when a low wire loop shape or short wire loop shape is made by the triangular loop formation process or the trapezoidal loop formation process. In particular, in cases where the amount of reverse movement in the reverse operation is sufficient so that a kink 3a is strongly formed in the wire 3 as shown in FIG. 6(b) and FIG. 7(b), an accurate triangular loop or trapezoidal loop can be formed as shown in FIG. 6(d) and FIG. 7(f); however, in cases where a low wire loop shape is to be formed, a large amount of reverse movement cannot be performed because the neck height part 31 must be low.
A wire shape in which a triangular wire loop formation process forms a low neck height part 31 is shown in FIG. 8, while a wire shape by a trapezoidal wire loop formation process that forms a low neck height part 31 is shown in FIG. 9.
In these cases, as shown in step (b) in FIG. 8 and in step (b) in FIG. 9, the kink 3a formed in steps (c) in FIGS. 6 and 7 cannot be obtained sufficiently, and a bent part 3c is formed in step (c) in FIGS. 8 and 9. As a result, a triangular loop or trapezoidal loop with an inaccurate shape which is bowed upward is formed and the overall height of the wire loop is increased in step (d) in FIG. 8 and in step (i) in FIG. 9.
Unfortunately, when the wire is formed with a bend in it initially or when a bend is formed in the wire during the wire loop forming operation, there is no way for removing such bends. As a result, bends and bows are generated in the bonded wires.